According to TErrestrial Trunked Radio (TETRA) standard, different modes of operating a subscriber unit in a communication system are known. Such modes of operation include, but are not limited to, a trunked mode operation (TMO) and a direct mode operation (DMO). In the TMO, subscriber units use network infrastructure, such as base stations, to communicate with each other. In the DMO, on the other hand, subscriber units communicate directly with other subscriber units within a limited coverage area, without using the network infrastructure. The DMO may be utilized to provide coverage in areas where the TMO network is not available. The DMO may be used within the coverage area of the TMO network, if a group of users wishes to communicate only internally and does not wish that the group communication is audible to all others communicating in the TMO network.
TMO systems generally provide improved geographic coverage and signal strength compared to a DMO system. However, there are several problems that can occur in systems communicating in a TMO only network. For example, the communications may be weak at the edges of the TMO network's coverage area or at the areas outside the TMO network's coverage area. As such, communication in TMO cannot be maintained when the subscriber unit, communicating in TMO, moves out of the coverage area of the TMO network, for example indoors, underground, rural, or in other circumstances with poor coverage. But, this is undesirable in situations where connections should always be maintained at all times. One such situation is an emergency situation where front line fire fighters are employed to respond to an incident in a building. The front line fire fighters may experience coverage issues when the TMO network is not available or when the front line fire fighters move out of the TMO network's coverage area. However, it is very important that the front line fire fighters always stay connected to their talk group at all times. So, it is desirable to extend the coverage of the TMO network such that the subscriber unit is able to continue to communicate even when the subscriber unit moves out of the coverage area of the TMO network.
Moreover, it is also desirable that the users do not have to manually intervene, turn knobs, press key buttons etc. to continue the communication. This is not convenient for some users such as fully equipped fire workers who do not want to bother themselves with the subscriber units during operation.
Accordingly, there is a need to provide improved communications services and to ensure that communications are not affected when a subscriber unit moves out of the coverage area of a TMO network.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present disclosure.
The method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present disclosure so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.